1. Field of the Invention
This invention relates generally to industrial ovens, and more particularly to methods and apparatus for heat treating semiconductor wafers.
2. Description of the Prior Art
Discrete semiconductor devices are formed in arrays on a surface of disk-shaped semiconductor wafers. After the devices are formed on a wafer, the wafer is cut into individual chips which are then packaged and sold.
A common step in the production of semiconductor devices is to heat the semiconductor wafer in an oven. Typically, twenty-five wafers are loaded on a cassette, and eight cassettes are placed within the oven.
In the prior art, the loaded cassettes are heated within vacuum ovens or within ovens filled with an inert gas to minimize reaction with the wafers. Vacuum ovens heat the wafers primarily by radiation and conduction, while inert gas ovens additionally heat the wafers by convection. Inert gas ovens usually heat the wafers more quickly due to their additional mode of heat transfer.
Inert gas ovens are first sealed and then flooded with an inert gas, such as nitrogen. Heating elements within the walls of the oven are energized to heat the nitrogen to operational levels. The heating elements in the walls of the oven also heat the shelves upon which the cassettes are resting, permitting heat to be conducted through the cassettes to the wafers that they are supporting.
A problem with conventional inert gas ovens is that the gas, which is usually stored under compression, is relatively cool when it enters the oven, having given up most of its heat energy in adiabatic expansion. In consequence, time is wasted heating the inert gas to operating temperatures, increasing the time that the wafers must remain within the oven.
Also, with increasing frequency, the semiconductor industry is using TEFLON cassettes to hold the semiconductor wafers due to their resistance to contamination and low reactivity with most materials. However, a problem encountered with a TEFLON cassettes is that they are very good heat insulators which all but eliminates heat conduction to the wafers from the shelves of the oven. Thus, TEFLON cassettes reduce the efficiency of inert gas ovens, and are even less suitable for vacuum ovens which rely heavily upon heat conduction from the shelves. The prior art response to the heat conduction problem of TEFLON is either to accept a longer heating time for the semiconductor wafers or transfer the wafers into another cassette for heating within the oven.